This invention relates to a GPS receiver comprising a GPS signal antenna for receiving externally transmitted GPS signals; an analogue-to-digital converter coupled to the antenna for sampling the received GPS signals; a memory for storing the GPS signal samples; and a digital GPS signal processor for retrieving pseudorange information from the GPS signal samples stored in the memory, wherein the receiver has a dormant mode of operation in which received GPS signals are sampled and stored in the memory but the signal processor is not operative for retrieving pseudorange information, and an active mode of operation in which the signal processor is operative for retrieving pseudorange information.
The invention further relates to a mobile unit incorporating such a GPS receiver and, in particular but not exclusively, to a mobile cellular telephone adapted to provide information corresponding to its position to the base station with which it is registered in the event of an emergency call being made.
At present, GPS is most notably associated with the Navigation System with Time and Ranging (NAVSTAR) GPS, an all weather, spaced based navigation system developed and operated by the US Department of Defense, however, the general principles underlying GPS are universal and not merely limited to NAVSTAR. Accordingly, GPS hereafter refers to any global positioning system comprising a plurality of radio transmitters at different locations and a receiver which determines its location based on the time of arrival and/or time difference of arrival of the transmissions of the radio transmitters.
As is well known, a GPS receiver having a digital GPS signal processor may implement a pseudorandom noise (PRN) code tracking loop in which early (E), prompt (P) and late (L) replica codes of satellite PRN codes are continuously generated, and compared to the incoming satellite PRN codes as received by the receiver. Assuming carrier phase lock, a linear code sweep should eventually result in the incoming PRN code being in phase with that of the locally generated replica and therefore, if detected, code acquisition. Once the code is acquired, the pseudorange information may be retrieved and using conventional navigation algorithms, the position of the receiver calculated. Recently, as an alternative to the above early-late correlation method, it has become known to use fast convolution methods and in particular, involving Fast Fourier Transforms (FFTs), in order to acquired the PRN codes. Such convolution methods are particularly suited to where fast PRN code acquisition is required and are further described in U.S. Pat. No. 5,781,156 which is incorporated herein by reference.
Notwithstanding which of the aforementioned techniques is used for code acquisition, the power consumption during signal acquisition and also during the position calculation process can be significant, up to approximately 1 watt. The problem is particularly relevant to handheld, battery powered GPS receivers which by their nature have a limited capacity and similarly to mobile telephones incorporating such receivers. In addition, with respect to mobile telephones, the problem is further compounded by the fashion for their miniaturisation for both aesthetic and ergonomic reasons.
An important reason for integrating GPS functionality in a mobile cellular telephone is that it has been proposed as a means to enable operators of cellular telephone networks to determine the location from which a call is made and, in particular, for an emergency call to the emergency services. Indeed, in the US, the Federal Communications Commission (FCC) has become the first regulatory authority to require operators to be able to do so. In the case of an emergency call, it is desirable for the positional information to be available as soon as possible, however, from a xe2x80x9ccold startxe2x80x9d where the GPS receiver does not have access to up to date ephemeris data or even worse from a xe2x80x9cfactory cold startxe2x80x9d where the GPS receiver does not have an up to date almanac, the time to first fix (TTFF) can be anywhere between 30 seconds and 5 minutes. Of course, the problems of cold starts can be eliminated if the GPS receiver is operative to provide a position fix continuously but this is undesirable for the aforementioned reasons relating to power consumption. Also, a user may simply wish to turn their telephone off yet may later wish to make an emergency call immediately after power up.
It is therefore an object of the present invention to provide a GPS receiver capable of the prompt and power efficient retrieval of pseudorange information from received GPS signals.
It is a further object of the invention to provide a mobile unit such as a mobile cellular telephone incorporating the same.
According to a first aspect of the present invention, a GPS receiver of the type described above is provided characterised in that the receiver (30) is arranged to change from the dormant mode to the active mode in response to a instruction received from external to the receiver (30) and made whilst the receiver (30) is operating in the dormant mode.
Such a GPS receiver, upon receiving an appropriate instruction, is able to immediately commence processing the signal samples stored in the memory in order to retrieve the pseudorange information and determining its current position. Furthermore, as receiving, sampling and storing the externally transmitted GPS signals are low power processes relative to the operation of the signal processor for retrieving pseudorange information, the overall power consumption of the GPS receiver remains relatively low.
To ensure that the most up to date GPS signal samples are stored in the memory, in the dormant mode, the received GPS signals may be sampled and stored in the memory continuously.
Alternatively, in the dormant mode, the received GPS signals may be sampled and stored in the memory periodically. This reduces the power consumption of the receiver whilst still enabling the processing of the stored signal samples to commence immediately in order to retrieve pseudorange information.
During use, there is no guarantee that the latest set of GPS signal samples stored in the memory will contain the necessary pseudorange information in order to calculate the position of the receiver. It may be that at the time the last sample set was taken, the GPS signals were received under circumstances of poor signal reception and for that there were insufficient GPS satellites in view from which to retrieve the necessary pseudoranges. For example, when caused by the GPS receiver being surrounded by tall buildings or foliage.
To mitigate the effects of this problem when the received GPS signals are sampled and stored in the memory periodically, the memory store may contain at least two separate sets of GPS signal samples, one separated in time from another. By doing so, it is more likely that at least one sets of samples will be of sufficiently strong GPS signals with a sufficient number of satellites in view. In short, the likelihood of retrieving pseudorange information without having to resample the GPS signals is increased.
In addition, by taking signal samples at different times, it is possible to double-check the determined position and also, if the signal samples are time-stamped, to identify and measure movement of the GPS receiver.
Operation in the dormant mode may be commenced in response to power up of the GPS receiver or, if the receiver comprises a user interface, in response to an instruction received from a user via the user interface.
Also, where the GPS receiver comprises a user interface and is already operating in the dormant mode, the change from the dormant mode to the active mode may be in response to an instruction received from a user via the user interface. This would enable the user to retrieve pseudorange information only when desired.
According to a second aspect of the present invention, a mobile unit is provided comprising a transmitter and a receiver adapted for two-way communication with a base station, and a GPS receiver according to the first aspect of the present invention.
The mobile unit may be arranged to change from the dormant mode to the active mode in response to an instruction received by the mobile unit from a base station. Also, the mobile unit may be further arranged to transmit positional information from the mobile unit to the base station.
In additional, the mobile unit may be a mobile cellular telephone for use with a cellular radio transmission system in the form of a plurality of base stations adapted for two-way radio communication and situated at respective geographical locations to define a corresponding plurality of overlapping service areas constituting one or more regions.
Such a mobile telephone would enable operators of cellular telephone networks to promptly determine the location of a mobile unit from which a call is made. NB. positional information includes not only direct expressions of the position of the mobile unit but also information from which this could be derived, for example, pseudorange information.
Furthermore, by sampling the GPS signal prior to activation of the active mode, say in response to an emergency call, the risk of signal interference and drowning of the GPS signal is reduced. Such signal interference and drowning may be caused by the mobile unit""s transmitter transmitting encoded voice or other data to a base station and perhaps also the mobile unit""s receiver receiving base station replies.
Operation in the dormant mode may commence upon the mobile telephone""s recognition of an emergency call being made and then to change to active mode in the event of a request being made by the emergency services operator. This would reduce power consumption whilst ensuring the sampled data was still available when required.
Alternatively, when operating in the dormant mode, to change from the dormant mode to the active mode may occur in response to recognition by the mobile telephone of an emergency call being made. This would enable the positional information to be available more quickly, possibly even in advance of connection of the mobile cellular telephone to the emergency services operator.
Recognition by the mobile telephone of an emergency call being made may include recognising when a user enters the first one or more digits of the emergency call telephone number, for example, by typing on a keypad or alternatively using speech recognition.
Operation of the dormant mode may also be in response to a change in the strength of the signals received from a base station of a cellular radio transmission system, e.g. indicative of the mobile telephone being removed from a pocket or briefcase; in response to detected movement of the mobile telephone, e.g. when detected by an accelerometer located in the mobile telephone; or in response to the retracting of a cover on the mobile telephone, e.g. in the case of a mobile telephone with a sliding cover protecting the key pad, by retracting the cover, or in the case of a xe2x80x9cflip topxe2x80x9d mobile telephone, by flipping the cover.
According to a third aspect of the present invention, a method of operating a GPS receiver of the type described above is provided comprising the steps of (i) in a dormant mode of operation, sampling and storing received GPS signals in the memory whilst the signal processor for retrieving pseudorange information is not operative; and (ii) in response to a instruction received from external to the GPS receiver, changing from the dormant mode to an active mode of operation in which the signal processor for retrieving pseudorange information becomes operative.
In the dormant mode, the received GPS signals may be sampled and stored in the memory continuously or alternatively periodically. Where they are stored periodically, the memory store may contain at least two separate sets of GPS signal samples, one separated in time from another. Also, where the GPS receiver further comprises a user interface, the transition from the dormant mode to the active mode may occur in response to an instruction received from a user via the user interface.